The present invention relates to power tongs of the type commonly used in the petroleum drilling industry for making up and breaking apart threaded connections in casing, tubing, drill pipe, and similar tubular products. In particular, the present invention relates to an open throat power tong with improved means for guiding rotational movement of the cage plate assembly with respect to the tong body.
Power tongs may be generally classified as open throat or closed throat power tongs. Closed throat power tongs generally have a circular ring member which totally encompasses the pipe joint during operation. An advantage of a closed throat or donut-type power tong design is that the tong body and gear will not easily spread, although such tongs have a serious disadvantage in that the tong cannot easily be laterally put on or taken off a string of pipe. Such tongs are shown, for example, in U.S. Pat. Nos. 3,507,174 and 3,635,105. Another version of a closed throat design utilizes a circular gear with a hinged portion, which enables the tong to be laterally put on or taken off the pipe. Such a tong is functionally a closed throat tong, however, because the gear completely encompasses the joint during the make-up or break-out operation. An example of such a closed throat design is shown in U.S. Pat. No. 4,215,602. Because of the difficulty in operation, the latter type of closed throat tong has not been widely accepted in the industry.
Many oil rig operators prefer an open throat power tong, which may be characterized by a partial (not fully circular) ring member. Although such tongs frequently utilize a hinged door to join sides of the tong body, the gear or ring typically has an open throat portion for receiving a pipe section. Thus, the door of an open throat tong may be opened and the throat of the gear aligned with the throat of the body so that the tong assembly can easily be put on or taken off a pipe. Such tongs are shown, for example, in U.S. Pat. Nos. 3,196,717, 4,170,907, and 4,346,629. A major operational advantage of an open throat design tong compared to a closed throat design tong is the ease with which the tong can be laterally moved on or taken off the pipe joint, although such tongs do suffer from various problems associated with "spreading" of the open throat, expecially during high torque operations.
Many types of open throat power tongs utilize a cage plate assembly containing a plurality of gripping heads. The cage plate assembly is typically rotatable both with respect to both the tong body and the rotary gear. Rotational movement of the cage plate assembly is generally guided by a plurality of guide rollers, each having a vertical axis. In one embodiment, the guide rollers are positioned in a groove provided in the top and bottom cage plates. This embodiment is shown, for example, in U.S. Pat. No. 4,084,453. In this case, the shaft for each roller assembly extends through the tong plate, and an annular recess is provided in the open throat rotary gear so that the cage plate assembly may move relative to the rotary gear. In another embodiment, the cage plate guide rollers may be positioned in an annular groove provided in the rotary gear, while the shaft for each roller extends through the upper and lower tong plates into a suitable recess provided in the cage plate assembly.
In either case, such cage plate guide rollers suffer from at least two basic problems: (1) the partial rotary gear is substantially weakened by the annular groove provided for either the guide roller or the vertical shaft and nut for the guide roller, and (2) the cage plate guide rollers experience excessive wear and frequently break because of excessive forces applied to the guide rollers during high torque operations as the partial rotary gear and/or tong plate spread.
A typical prior art solution to the problems associated with cage plate guide rollers in open throat design tongs has been to increase the cross-sectional area of the partial rotary gear, and to increase the size and strength of the cage plate guide rollers and their corresponding shafts. Although these solutions have somewhat alleviated the maintenance problems, they also have added to the costs of manufacturing the power tong and have added undesirable weight to the power tong assembly. Also, as petroleum recovery operations search deeper and deeper, higher torques are required to successfully make-up and break apart tubular joints, so that as spreading of the partial ring and tong plates increases, excessive wear and breakage of the cage plate guide rollers is becoming an increasing problem.
Still another problem in the performance of power tongs is a frictional loss between a rotating cage plate assembly and the upper and lower tong plates. In U.S. Pat. No. 4,084,453, for example, it may be seen that the upper portion of the cage plate assembly may be in physical contact with the upper tong body plate, which adds some frictional loss and detracts from the tong performance as the cage plate assembly rotates. Also, it may be seen in this same patent that the ends of the cage plate guide bearings opposite the retaining nut may be in engagement with the upper or lower surface of the annular groove in the cage plate assembly, which may take some of the cage plate assembly weight off the upper tong plate and thus reduce frictional losses between the cage plate assembly and the tong plates. The problem, however, is that roller-type guide bearings typically are not intended to absorb forces directed along the axis of the roller assembly. Also, when prior art tongs are operating at their highest torque potential, both spreading and bending forces are typically applied to the tong, so that a given guide roller bearing may be experiencing extreme horizontal forces on only one side of the roller and also may be experiencing large vertical forces on only a small portion of the end surface of the bearing. This combination tends to substantially decrease the life of the bearing assembly, or may result in breakage of the roller or roller shaft.
The above described frictional loss may possibly be minimized with a special pad between the cage plate assembly and the tong plate, as shown for example in U.S. Pat. No. 4,215,602. Also, U.S. Pat. No. 4,266,450 discloses the use of both a mounted roller in the annular groove of the cage plate assembly and a horizontally mounted roller riding on the groove base wall. Either of the solutions proposed in the above two patents, however, are believed to be unacceptable to the industry. Additional costs are clearly required for the power tong assembly according to the above two techniques, especially in the latter described patent. Moreover, these last two mentioned patents attempt to deal with the problem of frictional engagement between the cage plate assembly and the tong plate, but do not deal with either of the two basic problems associated with cage plate guide rollers described above, i.e., weakening of the partial ring and roller breakage.
There exists, therefore, a need for a power tong with improved means for guiding the rotation of the cage plate assembly with respect to the tong body, which will be reliable and not experience excessive wear or breakage during operation. Also, there exists a need for an open throat power tong with an improved means for guiding the rotational movement of the cage plate assembly, which does not significantly weaken the partial gear or does not suffer from the other disadvantages of the prior art.